purple flowers of globe alliums in a garden

Ornamental onions (Allium spp.) are grown from bulbs planted in the fall. Photo: C. Carignan, UME

Updated: March 6, 2023

Bulb is a term loosely used to include corms, tubers, tuberous roots, and rhizomes as well as true bulbs. They are broadly grouped into spring flowering (January to May) and summer flowering (June to September). Spring bulbs provide early color before most annuals and perennials bloom.

  • Popular spring bulbs include anemone, allium, crocus, tulip, narcissus (daffodils), and scilla.
  • Selecting high-quality spring bulbs is very important because the flower bud has already developed before the bulb is offered for sale. Size is important. Look for plump, firm bulbs. Also, check the root base to make sure this area is firm and intact.
  • Choose your bulbs on the basis of color and flower size for specific purposes. For example, small ones create a natural look and large ones stand out as specimen plants.
  • The most reliable tulips are the Darwin hybrids and species tulips such as Linifolia and Tarda. These tulips tend to live much longer and truly perennialize in the garden rather than many of the other cultivars that will live for a few years and then die off.
  • Crocus thomasianus is a tough durable crocus that is one of the best species to use in lawns or compacted areas such as urbanized soils or under trees with compacted soils. This vigorous species will easily naturalize, even in areas troubled by rodents.
  • Keep bulbs cool (60°- 65°F) until planting time, usually in October. Bulbs need time to establish a healthy root system prior to the onset of winter, therefore they should not be planted late.
  • Summer flowering bulbs include amaryllis, tuberous begonia, caladium, calla lily, colchicum (autumn crocus), dahlia, gladiolus, lycoris, lily, and spider lily. As with spring bulbs, select summer flowering bulbs that are large, firm, and plump. Before investing time and money, be aware that many of these bulbs are not cold hardy. They will have to be dug and properly stored in a frost-free location over the winter.

Site selection 

Most bulbs need full sunshine, so select a planting site that will provide at least five to six hours of direct sunlight a day. Bulbs left in the ground year after year should have eight to 10 hours of daily sunlight for good flowering.

  • Adequate drainage is an important consideration, as most bulb plants will not tolerate poor drainage and will rot easily if overwatered or planted in wet areas.
  • Function must also be kept in mind. If bulbs are being used to naturalize an area, toss the bulbs and plant them where they fall to create a scattered effect.
  • For individual planting holes, loosen the soil below the depth the bulb is to be planted. Add fertilizer and cover with a layer of soil to ensure the bulbs do not contact the fertilizers directly. Set bulb upright in planting hole and cover with amended soil.

Planting time

Hardy, spring-flowering bulbs are planted in late summer or early fall. Hardy, fall-flowering bulbs such as colchicum are planted in August. Tender, summer-flowering bulbs are planted in the spring after danger of frost. Lilies are best planted in late fall.

Depth of planting

It is best to check the correct planting depth for each bulb with a successful local grower or another good local source. Catalogs and reference books may be incorrect, depending on soil type and condition. Generally, plant bulbs two and one-half to three times the diameter of the bulb in depth. If you plant bulbs too shallowly, you may encourage frost heaving and lose the bulbs.


Normal rainfall usually provides enough moisture for bulbs, but during dry weather, water plants at weekly intervals, soaking the ground thoroughly. Don’t neglect your bulbs after they bloom.


Remove mulch as soon as the danger of severe freezing has passed in early spring. If mulch is left on the ground after growth starts, new shoots may be pale green or colorless and new stems and foliage may be broken.

Maintenance of bulbs


After plants bloom, fertilize them lightly, if needed, with a complete fertilizer (contains nitrogen, phosphorus, and potassium), following label directions. Avoid high nitrogen fertilizer. Be sure to keep fertilizer off the leaves and away from roots, as it will burn them.


Sometimes it is necessary to stake bulbs. Some bulb flowers will face in only one direction. Use the stake to orient the flower to face toward the front of the bed.


When flowers fade, cut them off to prevent seed formation. The development of seed pods takes stored food from the bulbs. Deadheading may also promote longer bloom periods. Avoid removing any foliage.


Bulb foliage is very important. Do not cut the leaves after flowering until they start to wither. This allows the green leaves to produce food for next year’s growth. After leaves turn yellow, cut and remove the stems and foliage of the plants to prevent diseases from infecting next year’s plants. Many of the spring-blooming bulbs are best moved in the fall after the foliage has disappeared, therefore you should use some sort of location marker in the spring so that you can locate the bulbs in the fall.

  • If bulbs have become crowded or need to be moved to another site, move only after the foliage has faded. Bulbs dug and moved before foliage fades are useless.
  • Divide bulbs carefully by digging up the entire clump. Separate the bulbs and replant them in other locations.

Tender bulbs

There are several bulbs or bulb-like plants that are not reliably hardy and should be protected or lifted from the soil for the winter. Dahlias, gladiolas, begonias, caladiums, crocosmias, and cannas are good examples of a tender bulb (many of these are actually corms, tubers, or roots).

Digging and storing

Many summer-flowering bulbs are tender and must be dug and stored, usually when the leaves on the plants turn yellow. Use a spading fork to lift the bulbs from the ground. Wash off any soil that clings to the bulbs, except those that are stored in pots or with the soil around them.

  • Cure them by spreading the washed bulbs in a shaded place to dry. Typically this takes one to three days. When they are dry, store them away from sunlight in a cool, dry basement, cellar, garage, or shed at 50° to 55°F. Avoid temperatures below 45°F or above 70°F. Be sure air circulates around stored bulbs.
  • Never store bulbs more than two or three layers deep as they generate heat which can cause decay. Leave the soil on achimenes, begonia, canna, caladium, and dahlia bulbs. Store these bulbs in clumps on a slightly moistened layer of peat moss or sawdust in a cool place. Rinse clean and separate them just before planting next spring.

Botanically speaking 

  • Bulb: a short, flattened stem bearing fleshy, food-storage leaves. Examples: lily, narcissus, and tulip.
  • Corm: a solid, vertical, underground stem with a bud on top and covered with dry leaf bases. Examples: crocus and gladiolus.
  • Tuber-corm-perennial: disc-shaped structures with buds on top and roots below. Examples: tuberous begonia, ranunculus, and anemone.
  • Tuberous root: a swollen root used for food storage. Example: dahlia.
  • Rhizome: thickened, underground stems that grow horizontally at or below the soil surface. Example: canna.

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